The distant universe is full of puzzling objects, and some of the most curious are the Lyman-Alpha Blobs (LABs), large clouds of hydrogen that glow brightly in ultraviolet. They are some of the largest individual objects known, and scientists finally know what’s making them shine.

An international team of astronomers has discovered two galaxies within one of the largest LABs, known as LAB-1. These two galaxies are undergoing an intense phase of star formation and the powerful light of the new stars is what is lightening up the surrounding gas.

“Think of a streetlight on a foggy night – you see the diffuse glow because light is scattering off the tiny water droplets,” lead author Jim Geach, from the University of Hertfordshire, said in a statement. “A similar thing is happening here, except the streetlight is an intensely star-forming galaxy and the fog is a huge cloud of intergalactic gas. The galaxies are illuminating their surroundings.”

LAB-1, found in 2000, was the first such blob ever discovered and has fascinated astronomers ever since. It is located 11.5 billion light-years from Earth and it is 300,000 light-years across, about three times the size of the Milky Way.

^{Optical image of LAB-1 the first Lyman-alpha blob ever discovered. ESO/M. Hayes}

These blobs are emitting their light at a particular ultraviolet wavelength known as Lyman alpha, but since they are so far away and the universe is expanding, the light becomes detectable at longer wavelengths. For this reason, the scientists used the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to investigate this object.

The paper, which is available online and will be published in the Astrophysical Journal, uses the data from ALMA and combines it with sophisticated simulations to demonstrate that the intense ultraviolet radiation created by newborn stars is the power lighting LAB-1.

“What’s exciting about these blobs is that we are getting a rare glimpse of what’s happening around these young, growing galaxies. For a long time the origin of the extended Lyman-alpha light has been controversial,” said Geach.

“But with the combination of new observations and cutting-edge simulations, we think we have solved a 15-year-old mystery: Lyman-alpha Blob-1 is the site of formation of a massive elliptical galaxy that will one day be the heart of a giant cluster. We are seeing a snapshot of the assembly of that galaxy 11.5 billion years ago.”